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Full text of "Handbook Of Chemical Engineering - I"

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396                              CHEMICAL ENGINEERING
but the hard work begins when the last traces of moisture are to be removed, that is when the material is wanted in a bone dry state.
PRACTICE OF DRYING
Organic Products.—Granulated sugar and table salt are usually dried in the rotary cylinder dryers heated by hot air. Sugar blocks are generally treated under vacuum. Organic extracts, milk, yeast, glues and gelatine, are handled in a vacuum-drum dryer, vacuum-ribbon dryer, and by the spray system. Glue and gelatin are also handled in chamber and tunnel dryers at low temperatures, which makes it necessary to cool the air artificially during the hot summer months.
Grain contains about 22 per cent moisture, and for storage this moisture must be reduced to about 10 per cent. The work is done on large drying floors and in towers, which are heated by hot air. A considerable saving of time and fuel can be accomplished by the use of rotary vacuum dryers. Textiles are generally handled in chamber and tunnel dryers, either in batches or continuous.
Rubber and electrical insulating material should be treated in standard chamber dryers or vacuum-shelf dryers, with or without the recovery of the so Ivents. Shel dryers are more expensive to install, but the saving in steam quickly pays for the extra expense. Explos'ves are handled in vacuum-shelf dryers, especially built for this purpose, with and without recovery of the solvents. The drying housing is equipped with large automatic doors which will open in case of explosion.
Paper and pulp are usually dried on atmospheric drum dryers heated by steam, and also in drying tunnels which are divided into several compartments that are kept at various temperatures.
Potatoes are first cooked, and the pulp is spread by rollers over the surface of large steam heated drying drums. Bituminous coal and all kinds of granular material are handled in tubular dryers. Soap is dried when in pieces in chamber dryers, and soap flakes are handled on ribbon dryers at low temperature. Starch, casein and similar products are handled in tunnel dryers and also in rotary vacuum apparatus. Food products, fruits and vegetables are usually dried by hot air in chamber dryers. Vegetables have also been handled under vacuum, which has given an excellent product.
Inorganic Products.—General rules for the application of certain types of dryers cannot be given as there are too many variations in the nature and quantity of the products to be handled. Usually rotary vacuum and drum dryers are used for delicate products, and direct-heated or steam-heated cylindrical rotary dryers are used for bulky materials that are not affected by higher temperatures. Chamber and shelf dryers (either atmospheric or vacuum) should be used for all delicate and sticky materials, also pastes, and in cases where the material is strongly acid or alkaline, as the trays that are placed on top of the shelves can be made of almost any kind of material which will prevent contamination of the product. Sticky material and paste can also be handled successfully on either a single or double roll drum dryer.
Waste Products.—Offal and tankage are generally handled in rotary steam jacketed dryers, in small quantities. Large plants use the tubular type, and the direct-heated dryer, and the concentrated stock coming from the tank-water evaporators is either added to the wet tankage, or dried separately on atmospheric drum dryers, so-called "stock rollers." Garbage which has first been degreased is usually handled in a direct heated dryer.
Beet pulp and brewers and distillers grain are treated in tubular rotary dryers with steam on the inside of the tubes. Beet pulp is also handled in direct heated cylinder dryers.